1. Field of the Invention
The invention relates to the fluidized catalytic cracking of vacuum residuum oil. More particularly, the invention relates to cracking vacuum residuum for a short contact time to produce a gasoline and lighter boiling fraction. This is accomplished by diluting the vacuum residuum with a cracked gas oil fraction.
2. Description of Other Related Methods in the Field
The cracking of a hydrocarbon by first injecting a clean gas oil, then dirtier gas oils at points along the reaction zone is shown in U.S. Pat. No. 2,908,630. This method of multiple injection was extended to the residual oil range in U.S. Pat. No. 3,193,494. In that patent a residual oil containing as much as 15 ppm by weight of nickel and 30 ppm by weight of vanadium was injected as the last component of a three component system. The first component was gas oil comprising 50-99% of the total feed and the second component was a heavier gas oil boiling from about 650.degree.-950.degree. F. The concentration of the residual oil was about one-tenth that of the first gas oil injected or 5-10% of the total feed. The use of diluents such as steam, nitrogen and hydrocarbons with boiling points less than about 430.degree. F. to improve the gasoline selectivity is shown in U.S. Pat. Nos. 3,617,496 and 3,617,497. U.S. Pat. No. 3,617,497 discusses cracking a gas oil by injecting a low molecular weight portion of the gas oil to the bottom of a riser and a separate higher molecular weight portion of the gas oil to the upper portion of a riser. Two articles describe a downstream injection system: Bryson, M. C. and Huling, G. P., Gulf Explores Riser Cracking, Hydrocarbon Processing, May, 1972, and Campagna, R. J. and Krishna, A. S., Advances in Resid Cracking Technology, Katalistiks Fifth Fluid Catalytic Cracking Symposium, May 22-23, 1984. The first of these articles utilizes the teaching of U.S. Pat. No. 3,617,497 and deals with conversion from bed cracking to riser cracking. The second article discusses vacuum gas oil (VGO) cracking and the use of alternate injection points to shift the gasoline/distillate ratio. The article states that the method causes a decrease in gasoline octane.